Data related to the biological systems of an individual can indicate a body state of an individual or a transition from one body state to another body state. Exemplary biological systems can include the circulatory system, respiratory system, nervous system, including the autonomic nervous system, or other similar biological systems. Accurate and quantifiable measurements of biological systems are useful in various applications to provide feedback to the individual on their current, historical or impending body state. In particular, functional or structural variations in cardiac activity can indicate biological system activity levels (e.g., parasympathetic and sympathetic activity levels of the autonomic nervous system), which can provide accurate measurements of a body state or a transition from one body state to another body state.